This invention relates to the preparation of modified solid fertilizers, and, in particular, to such fertilizers wherein the modifier is bound to the surface of the fertilizer.
Fertilizers are substances that contain major plant nutrients such as nitrogen, phosphorus, potassium, sulfur, or other elements in a form that is usable by plants. The fertilizer may be a solid or a liquid, and can be applied to the surface of the soil or below the surface. In one popular approach, the fertilizer is manufactured in granular form and provided to the consumer in bulk or in bags. After the fertilizer is applied, the nutrients dissolve into the soil to nourish the plants. The present invention relates primarily to solid fertilizers such as granular fertilizers.
As noted, the fertilizer has traditionally been composed primarily of major plant nutrients. More recently, it has become known to add modifiers to the solid fertilizer. Such modifiers can include, for example, minor nutrients (also called "micronutrients"), herbicides, pesticides, and nitrification inhibitors. In the latter case, a chemical slows the utilization by the plant of the nitrogen prevent in the fertilizer, so as to provide a dosage of nitrogen spread over a period of time rather than in a single large dose. There are a variety of mechanisms of nitrification inhibition, and correspondingly a number of types of chemicals that can be added to the solid fertilizer.
The modifiers can be provided to the fertilizer in two ways. In one, the modifier is mixed with the fertilizer composition, typically prior to granulation, so that the modifier is spread throughout the volume of each particle. In the other, the modifier is supplied to the surface of the particle. Mixing the modifier with the fertilizer composition prior to granulation has the disadvantages that the modifier may be exposed to a high temperature that could reduce its effectiveness, and also that more of the modifier may be used than is otherwise necessary.
Techniques have been developed for supplying the modifier to the surface of the solid fertilizer particles. The modifiers are typically in a very finely divided form. If the modifier is simply dusted onto the surface of the fertilizer particles, there is typically only limited adhesion and much of the modifier is lost to the fertilizer particles during mixing, bagging, distribution, or application to the soil. In a better approach, the finely divided modifier is caused to adhere to the surface of the fertilizer particles by adhesion promoters such as vegetable, animal, or mineral oils. This approach has the disadvantage that a foreign substance is necessarily provided to the soil, which may lead to damage to the plants or pollution.
U.S. Pat. No. 4,764,200 discloses an alternative approach to applying a nitrification inhibitor to the surface of a fertilizer particle. In this process, the finely divided nitrification inhibitor is applied to the surface of a sulfate-containing fertilizer particle, as in a rotating pan device. Aqueous calcium nitrate solution is thereafter sprayed onto the particles. The calcium reacts with the sulfate ions to produce a water-insoluble calcium sulfate binding agent that retains the nitrification inhibitor on the surface of the particles.
This approach may be useful on a laboratory scale, but the inventors have determined that, when applied in a commercial operation, it may have the important drawback that a great deal of the dust of the nitrification inhibitor is produced. This dust must be recovered, both because the nitrification inhibitor is expensive and also because it should not be released into the atmosphere. A solid recovery system is therefore required. Moreover, the operation requires two stages of application, the mixing of the nitrification inhibitor with the fertilizer particles, and spraying the aqueous calcium nitrate solution onto the particles. There may also be a tendency to agglomeration of the particles due to the hygroscopic nature of calcium nitrate. Finally, it is possible that incomplete application of the calcium nitrate solution can result in incomplete coating of the fertilizer particles.
There is needed an improved approach to the preparation of surface-modified solid fertilizers. The modifier should be well bound to the surfaces of the particles, and the production process should be effective and economic. The present invention fulfills this need, and further provides related advantages.